1. Field of the Invention
The present invention relates to a magnetic element, a magnetic information reproducing head for use in reproducing (reading) high-density magnetic information, and a magnetic information reproducing apparatus.
2. Description of the Related Art
Since the inception of a GMR head employing a giant magnetoresistance effect (GMR effect), recording density of magnetic recording has been increased at a rate of 100% per year. GMR elements include a spin-valve-type element, and an artificial-lattice-type element. The spin-valve-type element has a multilayer film including a ferromagnetic layer/nonmagnetic layer/ferromagnetic layer. The magnetization of one of the ferromagnetic layers is fixed by applying, e.g., an exchange bias magnetic field from an antiferromagnetic film, whereby the magnetization direction of the other ferromagnetic layer is reversed by an external magnetic field (signal magnetic field). Accordingly, a relative angle between magnetization directions of the two ferromagnetic layers changes, and this change can be detected as a change in the element resistance.
Spin-valve-type GMR elements include a CIP (Current In Plane)-type GMR element which detects change in resistance by applying electric current in plane to the multilayer film, and a CPP (Current Perpendicular to Plane)-type GMR element which detects change in resistance by applying electric current perpendicular to the multilayer film.
To cope with higher-density magnetic recording, TMR elements employing a tunnel magnetoresistance effect (TMR effect) have been developed. A TMR element has a multilayer film including a ferromagnetic layer/tunnel dielectric layer/ferromagnetic layer. When a voltage is applied between the two ferromagnetic layers, a tunnel current flows into the TMR element. A characteristic of a TMR element that the magnitude of a tunnel current flowing into the TMR element changes in accordance with the magnetization direction of the two ferromagnetic layers, can be utilized in detecting a change in the relative angle between the two ferromagnetic material layers as a change in tunnel resistance.
An MR ratio of a TMR element is approximately 50% maximum. Since a TMR element has an MR ratio larger than that of a GMR element, the signal voltage of a TMR element is larger than that of a GMR element. However, because of increased shot noise, a TMR element has not only larger pure signal components but also larger noise components than a GMR element, a TMR element has a problem that the SN ratio (signal-to-noise ratio) has not been improved.
Shot noise, which depends on current fluctuation caused by irregular passage of electrons through a tunnel barrier of a multilayer film, increases in proportion to the square root of the tunnel resistance. Therefore, to obtain a necessary signal voltage while suppressing shot noise, a tunnel dielectric layer must be made thinner so that the tunnel resistance is reduced.
As recording density becomes higher, the element size must be reduced so as to become substantially equal in size to a recording bit. To achieve this, a junction resistance of a tunnel dielectric layer must be diminished (i.e., the tunnel dielectric material layer must be made thinner). In order to attain a recording density of 300 Gbit/inch2 (gigabits/square inch), a required junction resistance must be 1Ωcm2 or smaller, which is equivalent to the thickness of a double-atomic layer of an Al—O (aluminum oxide film) tunnel dielectric layer. However, the thinner the tunnel dielectric layer, the more easily a short circuit can be created between the upper and lower ferromagnetic layers, which would result in a decrease in the MR ratio. Accordingly, fabrication of elements becomes exponentially more difficult.
In addition to the above, an element using spin-polarized current in a ferromagnetic material has been suggested. For example, in a spin injection three-terminal element, a transistor which performs gating by injecting spin-polarized current from a ferromagnetic material electrode into channels has been suggested (see JP-A-2002-26417).
Recently, a problem of magnetic white noise, which is common to above elements, has arisen. Unlike electrical noise such as shot noise, magnetic white noise is generated in response to thermal fluctuation of micro magnetic moment of a ferromagnetic material. As the downsizing of element proceeds, thermal fluctuation of elements is expected to become more dominant, exceeding the electrical noise in the range of 200 to 300 Gbit/inch2. Avoiding magnetic white noise and further increasing the recording density of magnetic recording requires development of a micro-magnetic sensor whose operation principle differs from that of the conventional magnetoresistance effects.